Werner Karl Heisenberg

Werner Karl Heisenberg
(1901-1976)

Physicist, philosopher, and public figure who helped to establish the
modern science of quantum mechanics, out of which came the famous indeterminacy,
or uncertainty, principle, for which he received the Nobel Prize for
Physics in 1932. He also made important contributions to the theories
of the hydrodynamics of turbulence, the atomic nucleus, ferromagnetism,
cosmic rays, and elementary particles; and he planned the first post-World
War II German nuclear reactor, at Karlsruhe, West Germany.

Introduction.
In his philosophical and methodological writings, Heisenberg was much
influenced by Niels Bohr and Albert Einstein. From the former he derived
the concepts of the social and dialogical character of scientific invention;
the principle of correspondence (pragmatic and model-theoretical continuity)
between macrophysics and microphysics; the permanence, though not the
universality, of classical physics; the "interactive," rather
than passive, role of the scientific observer in microphysics; and,
consequently, the contextualized character of microphysical theories.
From Einstein he derived the concepts of simplicity as a criterion of
the central order of nature; scientific realism (i.e., science describing
nature itself, not merely how nature can be manipulated); and the theory-ladenness
of scientific observations. He was coauthor with Bohr of the philosophy
of complementarity. In his later work he conceived of a central order
in nature, consisting of a set of universal symmetries expressible in
a single mathematical equation for all systems of particulate matter.
As a public figure, he actively promoted the peaceful use of nuclear
energy after World War II and, in 1957, led other German scientists
in opposing a move to equip the West German Army with nuclear weapons.
He was, in 1954, one of the organizers of the Conseil Europeen pour
la Recherche Nucleaire (CERN; later, Organisation Europeene pour la
Recherche Nucleaire) in Geneva.

Early life.
Heisenberg was born on December 5, 1901, in Wurzburg, Germany. He studied
physics, together with Wolfgang Pauli, his lifelong friend and collaborator,
under Arnold Sommerfeld at the University of Munich and completed his
doctoral dissertation (1923) on turbulence in fluid streams. Heisenberg
followed Pauli to the University of Gottingen and studied there under
Max Born; then, in the fall of 1924, he went to the Institute for Theoretical
Physics in Copenhagen to study under Bohr.
Heisenberg's interest in Bohr's model of the planetary atom and his
comprehension of its limitations led him to seek a theoretical basis
for a new model. Bohr's concept--after 1913 the centrepiece of what
has come to be called the old quantum theory--had been based on the
classical motion of electrons in well-defined orbits around the nucleus,
and the quantum restrictions had been imposed arbitrarily to bring the
consequences of the model into conformity with experimental results.
As a summary of existing knowledge and as a stimulus to further research,
the Bohr atomic model had succeeded admirably, but the results of new
research were becoming more and more difficult to reconcile with it.

In June 1925, while recuperating from an attack of hay fever on Helgoland,
an island in the North Sea, Heisenberg solved a major physical problem--how
to account for the stationary (discrete) energy states of an anharmonic
oscillator. His solution, because it was analogous to that of a simple
planetary atom, launched the program for the development of the quantum
mechanics of atomic systems. (Quantum mechanics is the science that
accounts for discrete energy states--as in the light of atomic spectra--and
other forms of quantized energy, and for the phenomenon of stability
exhibited by atomic systems.) Heisenberg published his results some
months later in the Zeitschrift fur Physik under the title "Uber
quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen"
("About the Quantum-Theoretical Reinterpretation of Kinetic and
Mechanical Relationships"). In this article he proposed a reinterpretation
of the basic concepts of mechanics.

Heisenberg's treatment of the problem departed from Bohr's as much
as Bohr's had from 19th-century tenets. Heisenberg was willing to sacrifice
the idea of discrete particles moving in prescribed paths (neither particles
nor paths could be observed) in exchange for a theory that would deal
directly with experimental facts and lead to the quantum conditions
as consequences of the theory rather than ad hoc stipulations. Physical
variables were to be represented by arrays of numbers; under the influence
of Einstein's paper on relativity (1905), he took the variables to represent
not hidden, inaccessible structures but "observable" (i.e.,
measurable) quantities. Born saw that the arrays obeyed the rules of
matrix algebra; he, Pascual Jordan, and Heisenberg were able to express
the new theory in terms of this branch of mathematics, and the new quantum
theory became matrix mechanics. Each (usually infinite-dimensional)
matrix of the theory specified the set of possible values for a physical
variable, and the individual terms of a matrix were taken to generate
probabilities of occurrences of states and transitions among states.
Heisenberg used the new matrix mechanics to interpret the dual spectrum
of the helium atom (that is, the superposed spectra of its two forms,
in which the spins of the two electrons are either parallel or antiparallel),
and with it he predicted that the hydrogen molecule should have analogous
dual forms. With others, he also addressed many atomic and molecular
spectra, ferromagnetic phenomena, and electromagnetic behaviour. Important
alternative forms of the new quantum theory were proposed in 1926 by
Erwin Schrodinger (wave mechanics) and P.A.M. Dirac (transformation
theory).

In 1927 Heisenberg published the indeterminacy, or uncertainty, principle.
The form he derived appeared in a paper that tried to show how matrix
mechanics could be interpreted in terms of the intuitively familiar
concepts of classical physics. If q is the position coordinate
of an electron (in some specified state), and p its momentum,
assuming that q, and independently, p have
been measured for many electrons (all in the particular state), then,
Heisenberg proved,

qp > h,

where q is
the standard deviation of measurements of q, p
is the standard deviation of measurements of p, and h
is Planck's constant (6.626176 10-27 erg-second). Indeterminacy principles
are characteristic of quantum physics; they state the theoretical limitations
imposed upon any pair of noncommuting (i.e., conjugate) variables, such
as the matrix representations of position and momentum; in such cases,
the measurement of one affects the measurement of the other. The enormous
significance of the indeterminacy principle is recognized by all scientists;
but how it is to be understood physically--whether it depends on using
intuitive classical ("complementary") pictures of a quantum
system, or whether it is a principle in (a new kind of quantum) statistics,
or whether in some sense through the special properties of the mathematical
model it also describes a character of individual quantum systems--has
been and still is much disputed. Bohr took the principle to apply to
the complementary pictures of a quantum system--as a particle or as
a wave pocket in classically intuited space; Heisenberg originally took
the principle to apply to the nonintuitive properties of quantum, as
distinct from classical, systems.

Bohr and Heisenberg elaborated a philosophy of complementarity to take
into account the new physical variables and an appropriate measurement
process on which each depends. This new conception of the measurement
process in physics emphasized the active role of the scientist, who,
in making measurements, interacted with the observed object and thus
caused it to be revealed not as it is in itself but as a function of
measurement. Many physicists, including Einstein, Schrodinger, and Louis
de Broglie, refused to accept the philosophy of complementarity.

Later life.
From 1927 to 1941 Heisenberg was professor at the University of Leipzig.
For the following four years, he was director of the Kaiser Wilhelm
(now Max Planck) Institute for Physics in Berlin. Although he did not
publicly oppose the Nazi regime, he was hostile to its policies. During
World War II he worked with Otto Hahn, one of the discoverers of nuclear
fission, on the development of a nuclear reactor. He failed to develop
an effective program for nuclear weapons, probably from want of technical
resources and lack of will to do so. After the war he organized and
became director of the Max Planck Institute for Physics and Astrophysics
at Gottingen, moving with the institute, in 1958, to Munich; he was
also, in 1954, the German representative for the organizing of CERN.
In the postwar period Heisenberg began working on a fundamental spinor
equation (a nonlinear differential equation capable of representing
with spinors--complex vectorlike entities--all possible particulate
states of matter). His intuitions had led him to postulate that such
an equation would exhibit a basic set of universal symmetries in nature
(a symmetry is a mathematic form invariant under groups of canonical
space-time and other changes in the representing elements), and be capable
of explaining the variety of elementary particles generated in high-energy
collisions. In this work, the "Platonic" character of which
he recognized, he had the support and collaboration of Hans-Peter Durr
and Carl Friedrich von Weizsacker.

Although he early, and indirectly, came under the influence of Ernst
Mach, Heisenberg, in his philosophical writings about quantum mechanics,
vigorously opposed the Logical Positivism developed by philosophers
of science of the Vienna Circle. According to Heisenberg, what was revealed
by active observation was not an absolute datum, but a theory-laden
datum--i.e., relativized by theory and contextualized by observational
situations. He took classical mechanics and electromagnetics, which
articulated the objective motions of bodies in space-time, to be permanently
valid, though not applicable to quantum mechanical systems; he took
causality to apply in general not to individual quantum mechanical systems
but to mathematical representations alone, since particle behaviour
could be predicted only on the basis of probability.

Heisenberg married Elisabeth Schumacher in 1937; they had seven children.
He loved music in addition to physics and saw a deep affinity between
these two interests. He also wrote philosophical works, believing that
new insights into the ancient problems of Part and Whole and One and
Many would help discovery in microphysics. Widely acknowledged as one
of the seminal thinkers of the 20th century, Heisenberg was honoured
with the Max Planck Medal, the Matteucci Medal, and the Barnard College
Medal of Columbia University. He died in Munich on February 1, 1976.

BIBLIOGRAPHY.
Books by Heisenberg include The Physical Principles of the Quantum Theory
(1930, reissued 1950; originally published in German, 1930), his most
important work, containing themes of early papers amplified into a treatise,
Philosophic Problems of Nuclear Science (1952, reissued 1966; originally
published in German, 8th enlarged ed., 1949), a collection of his early
essays, Physics and Philosophy: The Revolution in Modern Science (1958,
reissued 1989), his Gifford lectures, Physics and Beyond (1971; originally
published in German, 1969), a memoir of his early life, and Across the
Frontiers (1974, reissued 1990; orignally published in German, 1971),
collected essays and occasional lectures.
Biograpical material is found in Armin Hermann, Werner Heisenberg, 1901-1976,
trans. from German (1976); Carl Friedrich von Weizsacker and Bartel
Leendert van der Waerden, Werner Heisenberg (1977), in German; Elisabeth
Heisenberg, Inner Exile: Recollections of a Life with Werner Heisenberg
(1984; orginally published in German, 1980); and David C. Cassidy, Uncertainty:
The Life and Science of Werner Heisenberg (1992). Heisenberg's role
in the German wartime atomic program is chronicled in Leslie R. Groves,
Now It Can Be Told: Story of the Manhattan Project (1962, reprinted
1983). Collections of essays in honour of Heisenberg include Fritz Bopp
(ed.), Werner Heisenberg und die Physik unserer Zeit (1961); Heinrich
Pfeiffer (ed.), Denken und Umdenken: Zu Werk und Wirkung von Werner
Heisenberg (1977); and Peter Breitenlohner and H. Peter Durr (eds.),
Unified Theories of Elementary Particles (1982). Studies of Heisenberg's
philosophy of science include Patrick A. Heelan, Quantum Mechanics and
Objectivity (1965); and Max Jammer, The Philosophy of Quantum Mechanics:
The Interpretations of Quantum Mechanics in Historical Perspective (1974),
and The Conceptual Development of Quantum Mechanics, 2nd ed. (1989),
which provide the most complete study of Heisenberg's contribution to
quantum mechanics.